Behavior of streamers under divergent AC fields in transformer oils at large gaps

Results of investigations on prebreakdown phenomena in transformer oils under various divergent AC fields at large gaps are presented. The direct visualization of the light emitted by streamers was performed with a highly sensitive video device. Streamers of both polarities were observed, but it is found that positive streamers play a dominant role since their ability to propagate is much greater than that of the negative ones. Positive streamers can stop before they reach the opposite electrode, at a length determined mainly by their initiation voltage. Moreover, geometrical parameters (such as the gap distance and the shape and the size of both electrodes) are shown to have a significant influence, which is discussed in terms of capacitive coupling between the streamer filament and the electrodes